Field of Invention
This invention relates to circuits that operate at microwave frequencies particularly in the gigahertz range and more particularly to a field effect transistor mounted in a flip-chip carrier.
An oscillatory tendency of a field effect transistor (FET) is typically expressed in terms of Rollett's stability factor (k). See J. M. Rollet "Stability and Power-Gain Invariants of Linear Two-Ports", IRE Transactions on Circuit Theory, Vol. CT 9, pp. 29-32; March, 1962. When the factor is greater than unity, the FET is unconditionally stable.
When the stability factor is less than unity, the FET is potentially unstable. Because of this potential instability, a combination of a passive load impedance and a passive input impedance of the FET can be selected to induce oscillation. When the FET tends to oscillate at microwave frequencies, it can be used in two important types of microwave circuits: an oscillator and a reflection amplifier.
In both types of circuits, the FET inherently dissipates power in direct relation to power delivered to a load. Hence, the power delivered by the amplifier, for example, is limited by the amount of power that the FET can safely dissipate.
The FET typically has a gallium arsenide substrate with an N-type active region. As known to those skilled in the art, gallium arsenide has a high thermal resistance compared with the thermal resistance of metal used for a heat sink. Because of the high thermal resistance, it is difficult to conduct heat through the substrate to a heat sink. Therefore, the thermal resistance of the substrate may limit the power that the FET can safely dissipate.
Increased amounts of heat are conducted from the substrate by either making the substrate as thin as practical and plating it with metal or providing an increased spatial separation between elements of the FET. However, either plating the thin substrate or increasing the spatial separation of the elements increases the complexity of construction of the FET.
Alternatively, the heat may be conducted from the surface of the substrate where the heat is generated by mounting the FET in a type of heat sink known as a flip-chip carrier. The flip-chip carrier is referred to and shown in the article, "Thermal Resistance of GaAs Power FETs" by H. C. Huang, F. N. Sechi and L. S. Napoli in the Proceedings of the Sixth Biennial Cornell Electrical Enginerring Conference (1977). To understand the mounting in the carrier, it should be understood that the FET is comprised of a plurality of unit transistors with the substrate common to all of the unit transistors.
An exemplary unit transistor includes three elements, one of which is a thin metal deposition, known as a unit gate, that forms a Schottky barrier junction with the substrate. The other two elements are each a thin metal deposition in ohmic contact with the substrate. The other two elements serve as a unit drain and a unit source, respectively.
The total area on the substrate utilized by the unit sources is greater than the area utilized by either the unit gates or the unit drains. Therefore, the heat is most effectively conducted from the substrate when it is connected to the carrier through the unit sources. Moreover, this type of connection to the carrier does not include a wire lead, since the lead may introduce an undesired inductance.
The unit sources are easily connected to the carrier when they have a layer of metal plating thereby providing plated unit sources with surfaces that have a displacement from the substrate greater than the displacements of the surfaces of the unit gates and the unit drains. The FET is mounted with the surfaces of the plated unit sources in contact with a flat surface of the carrier. Because the surfaces of the plated unit sources has a greater displacement from the substrate than the surfaces of the unit gates and unit drains, the flat surface does not make contact with either the unit gates or the unit drains. The carrier is typically connected to a ground plane so that all of the unit sources form a grounded source electrode of the FET.
Usually, all of the unit drains are connected together by metal deposited on the substrate. Additionally, all of the unit gates are connected together by metal deposited on the substrate. However, the unit sources are not connected together by a metal deposition; they are only connected together through the carrier. When the substrate has an N-type active region, the unit gates and unit drains are biased negative and positive, respectively, relative to ground, to cause a bias current to flow from the unit drains to the unit sources. The unit gates, unit drains and unit sources are collectively referred to as a gate electrode, a drain electrode and a source electrode, respectively.
When the FET is mounted in the carrier, it is particularly suited for use in a common source circuit configuration because the unit sources form the grounded electrode. However, in the common source configuration, the FET usually has a stability factor greater than unity.
From the description given hereinbefore, it should be appreciated that the FET is not well suited for use in either reflection amplifiers or oscillators because the FET mounted in the carrier has a stability factor greater than unity. Heretofore, easily constructed power oscillators and reflection power amplifiers that utilize a field effect transistor mounted in a flip-chip carrier have been unknown in the art.